The present invention relates generally to solid state solar energy conversion cells. More specifically it relates to a process for fabricating multiple p-n junction devices from Czochralski silicon crystals.
Various solid state devices are known which are capable of converting light energy incident on the device into electrical energy. Two major problems with these devices have been the low efficiency of this conversion over the spectrum of sunlight and relatively high production costs.
Several devices of this type include a back surface field (BSF) cell and a vertical multi-junction (VMJ) cell. The BSF cell is discussed generally in J. G. Fossum, "Physical Operation of Back-Surface-Field Silicon Solar Cells ", IEEE Trans. on Electron Devices, Vol. ED-24, pp. 322-325 (1977). The VMJ cell is described in T. B. S. Chadda and M. Wolf, "Comparison of Vertical Multi-Junction and Conventional Solar Cell Performance", Proceedings of the 10th IEEE Photovoltaic Specialists Conference, pp. 52-57 (1974). The VMJ cells have improved efficiency and radiation hardness as compared to more conventional BSF cells, however, they are more difficult to fabricate. Also, these cells can be illuminated on only one surface.
It is therefore a principal object of the present invention to provide a process for fabricating multiple p-n junction devices and a device of this type which is an efficient solar energy conversion cell.
Another object is to provide a process whereby multiple p-n junctions are formed in a single silicon crystal.
Still another object is to provide a solar cell which is less costly to fabricate than known cells with comparable energy conversion efficiencies.
Yet another object is to provide a solar cell that is responsive to the full spectrum of sunlight, particularly long wavelength light, and can be illuminated at both major surfaces.